Methods of graph searching for border detection in image sequences with applications to cardiac magnetic resonance imaging.

Automated border detection using graph searching principles has been shown useful for many biomedical imaging applications. Unfortunately, in an often unpredictable subset of images, automated border detection methods may fail. Most current edge detection methods fail to take into account the added information available in a temporal or spatial sequence of images that are commonly available in biomedical image applications. To utilize this information the authors extended their previously reported single frame graph searching method to include data from a sequence. The authors' method transforms the three-dimensional surface definition problem in a sequence of images into a two-dimensional problem so that traditional graph searching algorithms may be used. Additionally, the authors developed a more efficient method of searching the three-dimensional data set using heuristic search techniques which vastly improve execution time by relaxing the optimality criteria. The authors have applied both methods to detect myocardial borders in computer simulated images as well as in short-axis magnetic resonance images of the human heart. Preliminary results show that the new multiple image methods may be more robust in certain circumstances when compared to a single frame method and that the heuristic search techniques may reduce analysis times without compromising robustness.

Task Force 1: The underserved.

The ACC has affirmed its commitment to universal access to health care. Underserved populations exist in urban and rural centers. Common to each is a paucity of personnel trained in cardiovascular care and a lack of access to preventive and highly technologic services. These factors contribute to a poor health outcome (75). Part of the rural problem can be corrected by the transfer of information to local providers by the use of new information systems. Included would be real-time electronic consultation, on-site subspecialty visits and the appropriate use of nonphysician providers (15). The urban problem requires changes in priorities and responsibilities of the academic health centers toward the communities they serve. Curricula changes of cardiovascular specialists, internists, generalists and nonphysician health care personnel must include diversity in training, physician training of ethnically matched providers in addition to technical excellence and research into methods of patient education and motivation for a healthier life-style (51). Reimbursement must appropriately reward those caring for underserved patients and those providing evaluation and management services (43,52).

Training in the care of adult patients with congenital heart disease.

The growing population of patients with congenital heart lesions surviving to adulthood necessitates a fresh look at the clinical training required to care for these patients. Physicians in pediatric and medical cardiology, general pediatrics, general medicine, family practice, obstetrics and gynecology and other specialities will all have a role in the care of these patients. Most likely, training will need to be incorporated into existing pathways with a clear delineation of a body of knowledge necessary to assimilate to complete the training program. The authors favor a certificate of added qualification for the treatment and management of adults with congenital heart disease to encourage excellence in preparation of physicians to care for this complex patient population.

Cardiac ultrasound. Progress and prospects.

Echocardiography is unique in its role as both a routine, mature technology and an expanding, innovative field. For the foreseeable future, ultrasound will remain one of the most important methods of diagnostic cardiac imaging.

Automated myocardial edge detection on MR images: accuracy in consecutive subjects.

The authors previously demonstrated the feasibility of graph-searching-based automated edge detection in cardiac magnetic resonance (MR) imaging. To further assess the clinical utility of this method, unselected images from 11 consecutive subjects undergoing clinically indicated (except for one healthy volunteer) short-axis spin-echo MR imaging were analyzed. A total of 142 images from the 11 subjects, encompassing the left ventricle from apex to outflow tract, were analyzed. The computer algorithm correctly identified complete endocardial and epicardial contours in 121 of 142 images (85%). Correlations between observer-traced and computer-derived epicardial areas for all images were good (r = .71 for epicardium, r = .83 for endocardium); they improved for a subset of higher-quality images (r = .82 for epicardium, r = .92 for endocardium). The authors conclude that the current data further support the usefulness of computer digital image processing in geometric analysis of cardiac MR image data.

Echocardiographic evaluation of isolated pulmonary valve disease in adolescents and adults.

Congenital pulmonary valve disease is often not discovered until adolescence or adulthood. Transthoracic two-dimensional echocardiography can provide detailed information regarding right ventricular outflow anatomy, although images are often less satisfactory than those obtained in infants and children. The more recent addition of biplanar transesophageal echocardiography has enhanced our ability to image the right ventricular outflow tract, pulmonary valve, and pulmonary artery noninvasively. Pulsed and continuous-wave Doppler estimates of subvalvular and transvalvular gradients have proved to be accurate. Doppler color flow mapping has proved useful in determining the location and direction of stenotic and regurgitant flow. With no accepted standard for comparison, quantification of regurgitation remains problematic. In many cases, echocardiography has replaced catheterization and angiography in the evaluation and long-term follow-up of congenital pulmonary valve disease before and after intervention.

Nonangiographic assessment of coronary artery bypass graft patency.

Coronary artery bypass graft patency can be assessed using the indirect techniques of evaluating patients' symptoms and exercise tolerance, changes in stress electrocardiogram, radioisotope regional perfusion, and myocardial wall contraction. The direct techniques assess graft patency directly by visualizing grafts using conventional computed tomography (CT), ultrafast CT, magnetic resonance imaging, digital subtraction angiography, and echocardiography. The advantages and disadvantages of each of these modalities are reviewed. At the present time, ultrafast CT and possibly magnetic resonance imaging and Doppler appear to be the only techniques besides angiography that can consistently evaluate bypass graft patency. Although they have the advantage of being minimally invasive, they cannot show graft stenosis or sequential graft patency. These techniques are best used in following patients after coronary bypass graft surgery and ruling out graft closure as the source of chest pain.

Multicenter trial of automated border detection in cardiac MR imaging.

The purpose of the present study was to evaluate the robustness of a method of automated border detection in cardiac magnetic resonance (MR) imaging. Thirty-seven short-axis spin-echo cardiac images were acquired from three medical centers, each with its own image-acquisition protocol. Endo- and epicardial borders and areas were derived from these images with a graph-searching-based method of edge detection. Computer results were compared with observer-traced borders. The method accurately defined myocardial borders in 36 of 37 images (97%), with excellent agreement between computer- and observer-derived endocardial and epicardial areas (correlation coefficients, .94-.99). The algorithm worked equally well for data from all three centers, despite differences in image-acquisition protocols, MR systems, and field strengths. These data suggest that a method of computer-assisted edge detection based on graph-searching principles yields endocardial and epicardial areas that correlate well with those derived by an independent observer.

Estimation of left ventricular cavity area with an on-line, semiautomated echocardiographic edge detection system.

BACKGROUND: Automated edge detection of endocardial borders in echocardiograms provides objective, reproducible estimation of cavity area; however, most methods have required off-line analysis. A recently developed prototype echocardiographic imaging system permits real-time automated edge detection during imaging and thus, the potential for measurement of cyclic changes in cavity area and the assessment of left ventricular function on-line. Our purpose was to compare measurements of endocardial area manually traced from conventional echocardiograms with those obtained with the real-time automated edge detection system in normal subjects. METHODS AND RESULTS: Two training sets of images were used to establish optimal methods of gain setting; the settings were then evaluated in a test set of images. In the high-gain training group (n = 8 subjects, 119 images), gain settings were adjusted sufficiently high to display at least 90% of the endocardial border. Manually drawn and real-time area measurements correlated at r = 0.92, but manually drawn areas were underestimated by computer. In the low-gain training group (n = 7 subjects, 104 images), gain settings were adjusted sufficiently low to avoid cavity clutter despite the presence of dropout of endocardial edges. Manually drawn and real-time areas again correlated (r = 0.79), but manually drawn areas were overestimated by computer. In the intermediate-gain test group (n = 7 subjects, 105 images), gain settings were balanced between maximal endocardial definition (greater than or equal to 90%) and minimal cavity clutter (less than or equal to 1 cm2). Manually drawn and real-time areas correlated at r = 0.91 for the group, and r ranged from 0.94 to 0.99 in individual subjects. Interobserver variability was 9.5% for manually traced areas and 10.6% for real-time area measurements. CONCLUSIONS: Real-time on-line automated edge detection provides accurate estimation of manually drawn cavity areas. Although the method is gain dependent, measurements are reproducible. The system should have clinical application in settings in which measurements of left ventricular function are important.

Effect of varying fibrinogen and hematocrit concentrations on magnetic resonance relaxation times of thrombus.

OBJECTIVES: The authors assessed the effects of varying one extracellular component (fibrinogen concentration) and one cellular component (hematocrit) on magnetic resonance (MR) T1 and T2 relaxation times of in vitro blood clots. METHODS: Blood from six male subjects was collected into sodium citrate anticoagulant (3.8%) and the whole blood was separated into platelet-rich plasma and packed erythrocytes. Subsequently, in vitro blood clots were made from varying concentrations of fibrinogen (1, 10, and 100 microM) in Tyrode's solution and washed, packed erythrocytes (hematocrit levels: 0%, 10%, 40%, and 80%). T1 and T2 measurements were completed at 20 MHz within 8 hours of initiating clotting. RESULTS: Significant shortening of MR relaxation times occurred with increasing fibrinogen concentration for hematocrit values of 0% and 10%. Extracellular fibrinogen concentration did not contribute significantly to variation in relaxation times at hematocrit values of 40% and 80%. For any given fibrinogen level, significant shortening occurred in T1 and T2 values for each successive increase in hematocrit values. CONCLUSIONS: Both extracellular (fibrinogen) and cellular (erythrocyte concentration) factors are significant determinants of thrombus T1 and T2 relaxation times.

Reproducibility of quantitative backscatter echocardiographic imaging in normal subjects.

Cyclic backscatter variation is useful in differentiating normal from ischemic and myopathic myocardium; however, there are few data on the reproducibility of clinical cyclic variation measurements. Therefore, a study using 2-dimensional and M-mode backscatter imaging was performed in 20 normal male subjects by 2 observers at an initial session and by 1 of the observers after 1 week. Cyclic variation on M-mode was calculated as the difference between the end-diastolic backscatter and the backscatter at the nadir. Two-dimensional determinations of backscatter were made using a single frame at end-diastole and one at end-systole. The cyclic change was the difference between backscatter measured in the end-diastolic and end-systolic frames. There were no statistically significant differences in analysis of variance among the grouped repeated measurements in either the interventricular septum or the posterior left ventricular wall. At the initial session, cyclic backscatter variation in the posterior wall using M-mode techniques was 5.9 +/- 1.8 dB (SD). The cyclic change in backscatter in the septal wall, using the 2-dimensional technique, was 4.3 +/- 2.4 dB. In the posterior wall, the cyclic change in backscatter was 5.7 +/- 1.7 dB. Pairwise observer correlations between repeated measurements ranged from -0.48 to 0.45. Thus, although there were no significant differences in group means on repeat measurements, repeated measurements in individual subjects were not reliably reproduced because of limited independent sampling of backscatter measurements at only 2 points in the heart cycle. Increased independent sampling and measurement from a backscatter waveform throughout the cardiac cycle may improve reproducibility of measurements.

NMR relaxation times in acute myocardial infarction: relative influence of changes in tissue water and fat content.

Tissue changes known to occur with acute myocardial infarction include increases in tissue water and lipid content. We sought to evaluate the relative contribution of alterations in tissue water and fat content to the changes of T1 and T2 relaxation times with infarction. Nine mongrel dogs underwent coronary artery occlusion for 6-12 h. T1 and T2 at 20 MHz and tissue water and fat content of normal and infarcted tissue were measured. Tissue water content, T1, and T2 were significantly greater in infarcted myocardium compared to normal (P less than 0.05). Tissue fat content, while not significantly different, increased linearly in infarcted samples as a function of duration of ischemia (r = 0.77). Despite this increase in fat content, only tissue water content was significantly linearly related to T1 (r = 0.97) and T2 (r = 0.91). Increases in T1 and T2 of infarcted tissue appeared to be most significantly influenced by changes in tissue water content. While total tissue fat content increased with duration of ischemia, it did not appear to significantly alter T1 or T2.